Methods of controlling and optimizing processes for producing chemical compounds are well known. The control of parameters such as temperature, pressure, mixing conditions, relative volumes of reactants, and uses of catalysts are generally well understood. Traditionally, newly discovered chemical compounds and processes involving either the production of such compounds, or processes involving the use of such compounds, have been initially carried out by researchers in “benchscale” environments. Particularly promising chemicals or processes may ultimately be produced in quantity by application to industrial-scale processes. Often, problems are encountered in scaling up a process from the laboratory to industrial-scale production.
Problems associated with moving from benchscale production to industrial-scale production often involve changes in process conditions between the benchscale environment and the industrial environment. For example, the temperature of the reactants in a small beaker or flask in a laboratory is much easier to keep constant than the temperature in a production tank having a capacity of hundreds of liters, as is often the case in a chemical processing plant. Variations in other process conditions within a large tank are also more difficult to control, and frequently affect the quality and yield of the desired product.
Another aspect of laboratory development of processes to produce chemical compounds is that often potentially dangerous chemicals are used to create the desired product. Fires and explosions in research laboratories and concomitant injury to personnel and property are well known risks in the chemical research industry. The risks are not limited only to research, since industrial chemical production facilities also may experience fires and explosions related to chemical production using dangerous chemicals. Often, due to the quantities of chemicals used in industrial-scale processes, such accidents are significantly more devastating in an industrial setting than similar accidents in a research setting.
Recently, much attention has been directed to the use of microscale reactors for both development and production of chemical processes. These types of reactors offer several advantages. As noted above, the control of chemical processes within very small reactors is much simpler than control in a large-scale production tank. Once a reaction process has been developed and optimized in a microscale reactor, it can be scaled up to industrial production level by replicating the microscale reactors in sufficient quantity to achieve the required production output of the process. If such reactors can be fabricated in quantity, and for a modest cost, industrial quantities of a desired product can be manufactured with a capital expenditure equal to or even less than that of a traditional chemical production facility. An additional benefit is that because the volume of material in each individual reactor is small, the effect of an explosion or fire is minimized, and with proper design, an accident in one reactor can be prevented from propagating to other reactors.
Safety in the research setting is also improved as such reactors generally require less exposure to hazardous substances and conditions by research personnel than traditional “batch chemistry,” which typically requires that the researcher physically handle chemicals in a variety of glass containers, often in the presence of a heat source. Any accident in such an environment is likely to increase the risk of exposure to hazardous chemicals, and cause significant damage to the laboratory. In contrast, small-scale or microreactors can be designed as self-contained units that minimize the researcher's potential exposure to chemical substances. When using a microreactor, the researcher is not required to physically manipulate containers of chemical materials to carry out a desired reaction, the reactor can be located in an area so that if an accident should occur, any resulting fire or explosion can be relatively easily contained.
Another area in which microreactors offer an advantage over conventional chemical process development and production is in the mixing of reactants. A mixing channel of the proper scale encourages a laminar flow of the reactants within the channel and is readily achievable in a microreactor. A laminar flow enhances mixing by diffusion, which eliminates the need to expend energy to physically stir or agitate the reactants and is an extremely fast and efficient mixing technique.
Microreactors are particularly applicable to the pharmaceutical industry, which engages in chemical research on many new chemical compounds every year, in the effort to find drugs or chemical compounds with desirable and commercially valuable properties. Enhancing the safety and efficiency of such research is valuable. When coupled with the potential that microreactors offer for eliminating the problems of moving from benchscale production to industrial production, it will be apparent that a microreactor suitable for use in carrying out a variety of chemical processes, and having an efficient and low cost design, will be in high demand.
Several different designs for microreactors have been developed. For example, such reactors are described in U.S. Pat. No. 5,534,328 and U.S. Pat. No. 5,690,763 (both listing Ashmead et al. as the inventors). These patents describe reactor structures for chemical manufacturing and production, fabricated from a plurality of interconnected layers. Generally, each layer has at least one channel or groove formed in it and most include orifices that serve to connect one layer in fluid communication with another. These layers are preferably made from silicon wafers, because silicon is relatively inert to the chemicals that may be processed in the reactor, and because the techniques required to mass produce silicon wafers that have had the required channels and other features etched into their surfaces are well known.
A disadvantage of the reactors described by Ashmead stems from the rather expensive and complicated process required for manufacturing the devices. While silicon wafer technology has advanced to the state that wafers having desired surface features can readily be mass produced, the equipment required is capital intensive, and unless unit production is extremely high, the substantial costs are difficult to offset. While Ashmead suggests that other materials can be used to fabricate the layers, such as metal, glass, or plastic, the surface features required (grooves, channels, etc.) must still be formed in the selected material. The specific surface features taught by Ashmead require significant manufacturing steps to fabricate. For instance, while forming an opening through a material is relatively easy, forming a groove or channel that penetrates only part way through the material comprising a layer is more difficult, as the manufacturing process must not only control the size of the surface feature, but the depth, as well. When forming an opening that completely penetrates through a material comprising a layer, depth control does not need to be so precisely controlled. Ashmead teaches that both openings, which completely penetrate the layers, and surface features (grooves/channels) that do not completely penetrate the individual layers are required. Hence, multiple processing steps must be employed in the fabrication of each layer, regardless of the material selected. Accordingly, it would be desirable to develop a microreactor comprising layers that do not require such detailed fabrication.
U.S. Pat. No. 5,580,523 (Bard) describes a modular microreactor that includes a series of modules connected in fluid communication, each module having a particular function (fluid flow handling and control, mixing, chemical processing, chemical separation, etc.). Bard specifically teaches that the plurality of modules are mounted laterally on a support structure, and not stacked, as disclosed by Ashmead. In a preferred embodiment of Bard, silicon wafer technology is again used to etch channels and/or other features into the surface of a silicon wafer. Other disclosed fabrication techniques include injection molding, casting, and micromachining of metals and semiconductor substrates. Again, the processing required to fabricate the individual modules goes beyond merely forming a plurality of openings into each component. Furthermore, the lateral layout of the reactor described by Bard requires a larger footprint (Basis Area) than a stacked plate reactor. In Bard's reactor, the more modules added, the larger the footprint of the entire reactor. In contrast, when additional plates are added to a stacked plate reactor, the footprint of the reactor does not change, which can be a distinct advantage, as in many work environments, the area an apparatus occupies on a workbench or floor is more valuable than the vertical height of the apparatus. It would be desirable to provide a reactor design that has a minimal footprint, while still providing the flexibility to add components to customize the reactor for a particular process or application.
In U.S. Pat. No. 5,961,932 (Ghosh et al.), a reactor is described that is formed from a plurality of ceramic layers, which are connected in fluid communication, at least one layer including a permeable partition. In particular, Ghosh describes the desirability of sizing fluid channels appropriately to provide for laminar flow and mixing via diffusion, rather than mixing via turbulence. In his preferred embodiment, Ghosh describes that channels and passageways are formed in each layer. The particular process Ghosh describes to accomplish this task involves fabricating the layers from “green” or uncured ceramic, which once shaped as desired, must be sintered. Significantly, the sintering process changes the size of the ceramic layer, so that the sizes of the features formed into the ceramic layer in the initial stages of production are different than in the finished product. It would be desirable to provide a reactor design in which the dimensions of the individual components can be rigidly controlled during fabrication, and are not subject to shrinkage, which can negatively effect the dimensions of the finished reactor. This object is particularly important when a reactor design focuses on achieving a laminar flow, because precise dimensional control of fluid pathways in the reactor must be maintained to achieve a consistent laminar flow, precisely controlled pressure drops, and precisely controlled fluidic resistance.
In all of these prior art reactors, relatively complicated manufacturing techniques are required. The manufacture of layers of silicon material requires a large capital investment. Sintering of a ceramic material requires the precise control of the shrinkage process, or individual components of a desired size cannot be achieved. In all cases, the prior art teaches that complicated structures (for example, fluid channels and reaction channels) must be etched or otherwise fabricated in each layer. Additionally, orifices or passages also need to be formed in each layer, so that fluids can move between adjacent layers of the reactor. Thus, a series of different manufacturing steps typically must be performed for each layer. It would be desirable to provide a reactor design offering the advantages described above, which is relatively simple to manufacture, so as to minimize capital investment in scaling up production from the laboratory to the industrial production levels.
As indicated above, while a single microreactor can produce only a limited volume of product, additional microreactors can be added in parallel to increase production capacity. When additional modular microreactor units are added, additional systems for reactant supply, heat transfer media supply, and product collection are typically required, which not only increases the complexity of the system, but also requires more space for duplicative fluid systems. Furthermore, even minor differences in feed rates for some of the duplicate reactor modules can negatively effect product quality. Finally, more sophisticated control and monitoring are required to manage additional reaction modules and feed systems. It would therefore be desirable to provide a microreactor capable of n-fold parallelization without requiring that additional fluid and control systems be provided.
In an array of identical fluid channels having a single common reactant distribution channel and a single common product collection channel, with the reactant inlet and the product outlet located at opposite ends, where the common reactant distribution and the common product collection channel have the same cross sectional area, if the viscosity of the product relative to the reactants is substantially the same, then the pressure drop through the array can be considered the same, and the resulting flow distribution is fairly even, with only slightly lower flow rates in the central fluid channels.
However, the flow distribution through such an array is not even if the viscosity of the product is significantly different than the viscosities of the reactants. When such an array is employed to process a reaction whose product has a significantly different viscosity compared to the viscosity of the mixture of the unreacted reactants, broad residence time distributions result in the array due to the fact that the pressure drop in the common reactant distribution channel no longer balances with the pressure drop in the common product collection channel. The flow rates within each individual fluid channel in the array are no longer identical. If the viscosity of the product is significantly greater than the viscosity of the mixed but unreacted reactants, then the flow rates in the individual fluid channels in the array tend to increase across the array for channels closest to the common product outlet. Thus the highest flow rate is experienced in the fluid channel in the array that is closest to the common product outlet, while the lowest flow rate is experienced in the fluid channel in the array that is located furthest from the common product outlet. This phenomenon is different if the viscosity of the product is less than the viscosity of the mixed but unreacted reactants. Thus for lower viscosity products the highest flow rate is experienced in the fluid channel in the array that is closest to the common reactant inlet, while the lowest flow rate is experienced in the fluid channel in the array that is located furthest from the common reactant inlet. The greater the relative change in viscosity, the greater the variation in flow rates across the array.
This imbalance leads to different residence times being associated with different fluid channels, resulting in an undesirable residence time distribution within the whole reaction unit. In certain cases, the additional residence time can lead to undesired cross reactions, and even clogging of the “slowest” fluid channels. It would be desirable to provide a microreactor including a plurality of fluid channels that is capable of processing reactant mixtures undergoing a significant viscosity change without the above-described residence time distributions and related problems.
It should further be noted that for the specific residence time distributions discussed above, relative to reactant mixtures produced in fluid channels in which a plurality of different reactants are mixed, only one type of undesirable residence time distribution is of concern. Residence time distribution problems of this type can also arise in fluid channels used to direct reactants before mixing, as well as products for collection. It would be desirable to provide a microreactor that includes a plurality of fluid channels adapted to provide substantially equal residence time distributions for fluid flow within the microreactor.
In summary, it is desirable to provide a microscale reaction apparatus that can be economically manufactured, can maintain a desired relatively narrow temperature range for a process, has a relatively modest footprint, can provide efficient diffusion mixing using a precisely controlled laminar flow, is capable of n-fold parallelization without requiring additional fluid supply, removal and control systems, can process reaction mixtures that form a product with a significantly different viscosity, and can provide substantially equal residence time distributions for fluid flow within the microreactor. Currently, the prior art does not include such apparatus.